Subnets. IP datagram format. The Internet Network layer. IP Fragmentation and Reassembly. IP Fragmentation & Reassembly. IP Addressing: introduction

Transcription

1 The Network layer Host, network layer functions: Network layer Routing protocols path selection R, OSPF, BGP Transport layer: TCP, forwarding table Link layer physical layer protocol addressing conventions datagram format packet handling conventions ICMP protocol error reporting signaling Network Layer 4-1 datagram format protocol version number header (bytes) type of data max number remaining hops (decremented at each ) upper layer protocol to deliver payload to how much overhead with TCP? 20 bytes of TCP 20 bytes of = 40 bytes + app layer overhead 32 bits head. type of ver len service fragment 16-bit identifier flgs time to upper header live layer checksum 32 bit source address 32 bit destination address Options (if any) data (variable, typically a TCP or segment) total datagram (bytes) for fragmentation/ reassembly E.g. timestamp, record route taken, specify list of s to visit. Network Layer 4-2 Fragmentation & Reassembly Fragmentation and Reassembly network links have MTU (max.transfer size) large datagram divided ( fragmented ) within net one datagram becomes several datagrams reassembled at final destination header bits used to identify, order related fragments reassembly fragmentation: in: one large datagram out: 3 smaller datagrams Example 4000 byte datagram MTU = 1500 bytes 1480 bytes in data field = 1480/8 =4000 =1500 =1500 =1040 One large datagram becomes several smaller datagrams =1 =1 =185 =370 Network Layer 4-3 Network Layer 4-4 Addressing: introduction address: 32-bit identifier for host, interface interface: connection between host/ and physical link s have multiple interfaces host typically has one interface address associated with each interface 7 = Subnets address: subnet part (high order bits) host part (low order bits) What s a subnet? can physically reach each other without intervening 7 subnet network consisting of 3 subnets Network Layer 4-5 Network Layer 4-6 1

2 Subnets How many? addressing: CR CR: Classless InterDomain Routing subnet portion of address of arbitrary address format: a.b.c.d/x, where x is # bits in subnet portion of address subnet host part part Network Layer 4-7 Network Layer 4-8 addresses: how to get one? Q: How does a host get address? hard-coded by system admin in a file : Dynamic Host Configuration Protocol: dynamically get address from as server plug-and-play : Dynamic Host Configuration Protocol Goal: host gets its address from server when it joins network Can renew its lease on address in use Allows reuse of addresses overview: host broadcasts discover msg [optional] server responds with offer msg [optional] host requests address: request msg server sends address: ack msg Network Layer 4-9 Network Layer 4-10 client-server scenario A B server 7 E arriving client needs address in this network client-server scenario server: discover src : , 68 dest.: ,67 yiaddr: transaction : 654 offer src: , 67 dest: , 68 transaction : 654 Lifetime: 3600 secs request src: , 68 dest:: , 67 transaction : 655 time Lifetime: 3600 secs ACK src: , 67 dest: , 68 transaction : 655 Lifetime: 3600 secs arriving client Network Layer 4-11 Network Layer

3 : more than address can also return : address of first-hop for client name and address of DNS sever network mask (indicating network versus host portion of address) : example (runs ) connecting laptop needs its address, addr of firsthop, addr of DNS server: use request encapsulated in, encapsulated in, encapsulated in ernet ernet frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at running server ernet demuxed to demuxed, demuxed to Network Layer 4-13 Network Layer 4-14 : example (runs ) DCP server formulates ACK containing client s address, address of first-hop for client, name & address of DNS server encapsulation of server, frame forwarded to client, demuxing up to at client client now knows its address, name and address of DSN server, address of its first-hop addresses: how to get one? Q: How does network get subnet part of addr? A: gets allocated portion of its provider ISP s address space ISP's block /20 Organization Organization /23 Organization / Organization /23 Network Layer 4-15 Network Layer 4-16 Hierarchical addressing: route aggregation Hierarchical addressing allows efficient advertisement of routing information: Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organization 1 Organization 0 Organization 0 Organization /23 Organization / Organization /23 Fly-By-Night-ISP ISPs-R-Us / /16 Organization / Organization /23 Organization /23 Fly-By-Night-ISP ISPs-R-Us / /16 or /23 Network Layer 4-17 Network Layer

4 addressing: the last word... Q: How does an ISP get block of addresses? A: ICANN: Corporation for Assigned Names and Numbers allocates addresses manages DNS assigns domain names, resolves disputes rest of All datagrams leaving local network have same single source NAT address:, different source port numbers local network /24 Datagrams with source or destination in this network have /24 address for source, destination (as usual) Network Layer 4-19 Network Layer 4-20 Motivation: local network uses one external address: range of addresses not needed from ISP: just one address for all devices can change addresses of devices in local network without notifying outside world can change ISP without changing addresses of devices in local network devices inside local net not explicitly addressable, visible by outside world (a security plus). Network Layer 4-21 Implementation: NAT must: outgoing datagrams: replace (source address, port #) of every outgoing datagram to (NAT address, new port #)... remote clients/servers will respond using (NAT address, new port #) as destination addr. remember (in NAT translation table) every (source address, port #) to (NAT address, new port #) translation pair incoming datagrams: replace (NAT address, new port #) in dest fields of every incoming datagram with corresponding (source address, port #) stored in NAT table Network Layer : NAT changes datagram source addr from, 3345 to, 5001, updates table 2 NAT translation table WAN side addr LAN side addr, 5001, 3345 S:, 5001 D: , 80 S: , 80 D:, : Reply arrives dest. address:, 5001 S:, 3345 D: , 80 1 S: , 80 D:, : host sends datagram to , : NAT changes datagram dest addr from, 5001 to, bit port-number field: 60,000 simultaneous connections with a single LAN-side address! NAT is controversial: s should only process up to layer 3 violates end-to-end argument NAT possibility must be taken into account by app designers, e.g., P2P applications address shortage should instead be solved by v6 Network Layer 4-23 Network Layer

5 NAT traversal problem client wants to connect to server with address server address local to LAN (client can t use it as destination addr) only one externally visible NATed address: solution 1: statically configure NAT to forward incoming connection requests at given port to server e.g., ( , port 2500) always forwarded to port Client? NAT NAT traversal problem solution 2: Universal Plug and Play (UPnP) Allows NATed host to: learn public address () add/remove port mappings (with lease times) NAT IGD Network Layer 4-25 Network Layer 4-26 NAT traversal problem ICMP: Control Message Protocol solution 3: relaying (used in Skype) NATed client establishes connection to relay External client connects to relay relay bridges packets between connections Client 2. connection to relay initiated by client 3. relaying established 1. connection to relay initiated by NATed host NAT communicate network-level information error reporting: unreachable host, network, port, protocol echo request/reply (used by ping) network-layer above : ICMP msgs carried in datagrams ICMP message: type, code plus first 8 bytes of datagram causing error Type Code description 0 0 echo reply (ping) 3 0 dest. network unreachable 3 1 dest host unreachable 3 2 dest protocol unreachable 3 3 dest port unreachable 3 6 dest network unknown 3 7 dest host unknown 4 0 source quench (congestion control - not used) 8 0 echo request (ping) 9 0 route advertisement 10 0 discovery 11 0 TTL expired 12 0 bad header Network Layer 4-27 Network Layer 4-28 Traceroute and ICMP Source sends series of segments to dest first has TTL =1 second has TTL=2, etc. Unlikely port number When nth datagram arrives to nth : discards datagram and sends to source an ICMP message (type 11, code 0) ICMP message includes name of & address when ICMP message arrives, source calculates RTT traceroute does this 3 times Stopping criterion segment eventually arrives at destination host destination returns ICMP port unreachable packet (type 3, code 3) when source gets this ICMP, stops. Network Layer